The present invention relates generally to radio telephones and, more particularly, to enhancing the gain of a portable radio telephone during hands-free operation.
Radio frequency (RF) telephones, commonly and hereinafter referred to as cellular telephones, have come into wide use in recent years. As cellular telephone technology advanced, the cellular telephone became much smaller and more portable. Instead of mounting the cellular telephone permanently in a vehicle or within a pouch resembling a small briefcase, cellular telephones are carried on the person and easily slip into a pocket of an article of clothing or a purse. Given the enhanced portability of these cellular telephones, various types of docking adapters, commonly referred to as cradles, are available for the portable telephone to rest in for hands-free operation and storage in an automobile.
Generally, the cradles interface with the portable cellular telephones to recharge the telephone's batteries and connect to an antenna mounted on the car's exterior, among other functions. A primary concern in the present invention is providing a portable cellular telephone capable of interfacing with a cradle for hands-free operation while driving. Automobile cradles providing hands-free operation preferably include, or are associated with, an external speaker and microphone. Hands-free headsets having a microphone and speaker are also available.
Currently, portable cellular telephones are given an EIA Station Class Mark which corresponds directly to various class levels. Each class level represents the maximum amount of output power allowed when transmitting radio frequencies for a particular type of cellular telephone. For example, Class 1 telephones include car telephones having a maximum output power up to 4 watts and Class 3 telephones include hand-held portable telephones having a maximum output power up to 1.2 watts. Class 3 telephones typically operate at approximately 0.6 watts while typical Class 1 operating power is 3 watts.
When using Class 3 portable cellular telephones with currently available automotive cradles, several RF problems affect efficient communications in the up-link to the cellular base station and the down-link to the portable cellular telephone. Most notable of these problems is that the metal body and frame of the automobile acts as an RF shield, and therefore, attenuates transmission of RF signals to and from the portable cellular telephone when operated from within the vehicle. This problem is typically overcome by using an external antenna mounted on the outside of the automobile and associated with the automobile cradle through a series of RF cables and connectors. An internal RF switch which switches from the telephone's internal antenna to the external antenna associated with the cradle when the telephone is placed within the automotive cradle is included in most portable cellular telephones.
When placed in a cradle coupled to an external antenna, the portable cellular telephone is able to avoid the substantial RF losses associated with the automobile shielding. The additional cabling and connectors associated with the external antenna introduce smaller, yet significant, RF losses, thus precluding the full benefit of using an external antenna.
Prior to the present invention, cellular telephone and cradle manufacturers overcame the problems outlined above in various ways. Most portable cellular telephones use a direct connection or a reactive coupling to provide an RF interface with the external antennas. These interfaces allow for the use of external power boosters placed in series between the portable telephone's RF interface and the external antenna to increase the RF up-link transmission power from Class 3 to Class 1. The boosters increase the power up to 3 watts, which is significantly above the Class 3 maximum of 1.2 watts and well within Class 1 operating parameters. Some currently available boosters provide a low noise amplifier in the down-link receiving path to enhance receiver sensitivity and path matching. Path matching generally refers to providing substantially the same effective power for the up-link and down-link transmission paths between the telephone and a base station.
Another technique for overcoming the above mentioned problems is disclosed in the Motorola patent by Mitzlaff, U.S. Pat. No. 4,636,741. Mitzlaff discloses an approach which changes a Class 3 portable cellular telephone to a Class 1 telephone when using an automotive cradle. In particular, Mitzlaff uses a telephone with a power amplifier which operates at two different power levels. The first set of power levels is used during portable operation of the cellular telephone while apart from the cradle. The second set of higher power levels is used when the portable cellular telephone interfaces the cradle and uses an external power source having a higher voltage than the telephone's internal battery. Mitzlaff requires changing the portable cellular telephone Station Class Mark from Class 3 to Class 1 when an external power source or cradle is detected.
These solutions have significant drawbacks. Both solutions increase the Station Class Mark and add substantial expense to the telephone, cradle or both. Both external boosters and Mitzlaffs amplifier configuration amplify the transmit signal from the Class 3 rating of 0.6 watts to a Class 1 rating of 3 watts. In most situations, an increase of this magnitude is not necessary to maintain a call when traveling between cells. During normal operations, the base station will signal the cellular telephone to step up or down the telephone's transmitting power to increase or lower transmission power to an acceptable minimum level. Thus, the command from the base station nullifies the booster's or internal amplifier's additional power amplification. In most situations, adding an amount of amplification to compensate for the RF losses of the RF cable and connectors extending between the cradle and the external antenna is all that is necessary to maintain a call. Quintupling the transmitting power and changing Station Class Mark ratings is economically impractical and operational overkill.
Thus, there remains a need for a portable cellular telephone capable of interfacing with a cradle and overcoming RF losses associated with the use of an external antenna during hands-free operation without requiring specialized high-output internal power amplifier circuitry or external boosters. Furthermore, there is a need to overcome the RF losses associated with the external antenna without requiring a change in the portable cellular telephone's Station Class Mark.